Anode assembly for cathodic protection

ABSTRACT

The cathodic protection of a reinforced concrete structure utilizes sacrificial anodes such as aluminum or zinc as well as alloys thereof. Each anode is embedded or substantially covered in a material consisting of a hydrophilic non-cementious open-cell foam. An activating agent such as one or more lithium salts is contained within the cells of the foam to maintain the anodes in an electrochemically active state. The activating agent may be immobilized in the cells using an aqueous gel such as agar. One or more metallic conductors electrically connect the anodes to the metal reinforcing members.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to provisional patent application Ser. No. 61/849,291, filed on Jan. 24, 2013, entitled “ANODE ASSEMBLY FOR CATHODIC PROTECTION which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention generally relates to the field of galvanic cathodic protection of steel embedded in concrete structures, and is particularly concerned with the performance of embedded sacrificial anodes, such as zinc, aluminum, and alloys thereof.

2. Description of Prior Art

The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to “passivate” such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt (NaCl) over a number of years results in the concrete becoming contaminated with chloride ions. Salt is commonly introduced in the form of seawater, set accelerators, or deicing salt.

When the chloride reaches the level of the reinforcing steel, and exceeds a certain threshold level for contamination, it destroys the ability of the concrete to keep the steel in a passive, non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of the steel can occur. The products of corrosion of the steel occupy 2.5 to 4 times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, coupled with traffic pounding, the utility or integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate. In a 2011 report to Congress, the Federal Highway Administration reported that of the nation's 605,086 bridges, about 145,000 (24% of the total) were classified as either functionally or structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.

Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques, such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt-contaminated concrete.

Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This cathodic polarization of the steel tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several different types of anodes have evolved for specific circumstances and different types of structures.

The most commonly used type of cathodic protection system is impressed current cathodic protection (ICCP), which is characterized by the use of inert anodes, such as carbon, titanium suboxide, and most commonly, catalyzed titanium. ICCP also requires the use of an auxiliary power supply to cause protective current to flow through the circuit, along with attendant wiring and electrical conduit. This type of cathodic protection has been generally successful, but problems have been reported with reliability and maintenance of the power supply. Problems have also been reported related to the durability of the anode itself, as well as the concrete immediately adjacent to the anode, since one of the products of reaction at an inert anode is acid (H⁺). Acid attacks the integrity of the cement paste phase within concrete. Finally, the complexity of ICCP systems requires additional monitoring and maintenance, which results in additional operating costs.

A second type of cathodic protection, known as galvanic cathodic protection (GCP), offers certain important advantages over ICCP. GCP uses sacrificial anodes, such as zinc and aluminum, and alloys thereof, which have inherently negative electrochemical potentials. When such anodes are used, protective current flows in the circuit without need for an external power supply since the reactions that occur are thermodynamically favored. GCP therefore requires no rectifier, external wiring or conduit. This simplicity increases reliability and reduces initial cost, as well as costs associated with long term monitoring and maintenance. Also, the use of GCP to protect high-strength prestressed steel from corrosion is considered inherently safe from the standpoint of hydrogen embrittlement. Recognizing these advantages, the Federal Highway Administration issued a Broad Agency Announcement (BAA) in 1992 for the study and development of sacrificial anode technology applied to reinforced and prestressed bridge components. As a result of this announcement and the technology that was developed because of this BAA, interest in GCP has greatly increased over the past few years.

In PCT Published Application WO94/29496 and in U.S. Pat. No. 6,022,469 by Page, a method of galvanic cathodic protection is disclosed wherein a zinc or zinc alloy anode is surrounded by a mortar containing an agent to maintain a high pH in the mortar surrounding the anode. This agent, preferably lithium hydroxide (LiOH), serves to prevent passivation of the zinc anode and maintain the anode in an electrochemically active state. In this method, the zinc anode is electrically attached to the reinforcing steel causing protective current to flow and mitigating subsequent corrosion of the steel.

In expired U.S. Pat. No. 5,292,411 Bartholomew et al discloses a method of patching an eroded area of concrete comprising the use of a metal anode having an ionically conductive hydrogel attached to at least a portion of the anode. In this patent it is taught that the anode and the hydrogel are flexible and are conformed within the eroded area, the anode being in elongated foil form.

In U.S. patent application Ser. No. 08/839,292 filed on Apr. 17, 1997 by Bennett, abandoned in favor of CIP application now U.S. Pat. No. 6,217,742 B1, the use of deliquescent or hygroscopic chemicals, collectively called “humectants” is disclosed to maintain a galvanic sprayed zinc anode in an active state and delivering protective current. In U.S. Pat. No. 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO₃ and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. And in U.S. Pat. No. 6,217,742 B1, issued Apr. 17, 2001, Bennett discloses the use of LiNO₃ and LiBr to enhance the performance of embedded discrete anodes. The entire subject matter of these three patents and abandoned application is incorporated herein by reference in full. And finally, in U.S. Pat. No. 6,165,346, issued Dec. 26, 2000, Whitmore discloses the use of deliquescent chemicals with the apparatus disclosed by Page in U.S. Pat. No. 6,022,469.

In U.S. Pat. No. 7,160,433 B2, issued Jan. 9, 2007, a method of cathodic protection of reinforcing steel is disclosed comprising a sacrificial anode embedded in an ionically conductive compressible matrix designed to absorb the expansive products of corrosion of the sacrificial anode metal. And in U.S. Pat. No. 8,157,983 B2, issued Apr. 17, 2012, a covering material incorporating a compressible water-retaining mineral in exfoliated form surrounding the anode is disclosed. The subject matter of both of these patents is incorporated herein by reference in full.

In U.S. Pat. No. No. 6,572,760 B2, issued Jun. 3, 2003, Whitmore discloses the use of a deliquescent material bound into a porous anode body, which acts to maintain the anode electrochemically active, while providing room for the expansive products of corrosion. The same patent discloses several mechanical means of making electrical connection to the reinforcing steel within a hole drilled into the concrete covering material. Many of these means involve driven pins, impact tools, and other specialized techniques, with attendant drawbacks. In U.S. Pat. No. 6,193,857, issued Feb. 27, 2001, Davison, et al describes an anode assembly comprising a block of anode material cast around an elongated electrical connector (wire). It further discloses making contact between the elongated connector and the reinforcing steel by winding the connector around the reinforcing steel and twisting the ends of the connector together using a twisting tool.

Finally, in U.S. Pat. No. 7,488,410 B2, issued Feb. 10, 2009, an anode assembly for cathodic protection is disclosed in which a non-conductive barrier is disclosed that covers one side of the anode assembly. The purpose of the non-conductive barrier is to reduce the passage of current to the adjacent portion of the reinforcing member to which the anode assembly is attached. The subject matter of this patent is incorporated herein by reference in full.

The anodes described above and the means of connection disclosed have become the basis for commercial products designed to extend the life of patch repair and to cathodically protect reinforced concrete structures from corrosion. But some embodiments, such as the use of high pH to maintain the anode in an electrochemically active state as described by Page, result in protective current that is small and often inadequate to mitigate corrosion. Use of the chemicals disclosed by Bennett, such as lithium nitrate and lithium bromide, result in a higher current. In cases of high chloride contamination and the presence of strong corrosion of the reinforcing steel, higher protective current is desirable, and sometimes necessary to prevent corrosion of steel embedded in concrete.

Also, some of the chemicals used to maintain the zinc anode in an electrochemically active state render the corrosion products of zinc largely insoluble. In this case the expansive corrosion products apply stress to the surrounding concrete, and when this stress exceeds the tensile strength of the concrete, cracking of the concrete can occur. Although several potential solutions have been proposed, including the ionically compressible conductive matrix described in U.S. Pat. No. 7,160,433, cracking remains a problem in some cases.

Cracking of the concrete overlay has been largely overcome by the addition of a compressible, ionically conductive phyllosilicate mineral such as vermiculite, as described in Bennett U.S. Pat. No. 8,157,983 B2. In this patent, the subject matter of which is incorporated herein by reference in full, vermiculite particles in the matrix appear to serve both functions of increasing the protective current delivered by the anode, and effectively absorbing the expansive products of corrosion.

It is noteworthy that, although one or two patent applications in this field have been broadly worded, there have been no commercial applications or reported positive results utilizing anything other than a cementitious covering material for the sacrificial anode.

SUMMARY OF THE INVENTION

The present invention relates to an anode assembly for cathodic protection of reinforced concrete, and more particularly, to a method for improving the performance and service life of embedded anodes prepared from sacrificial metals such as zinc, aluminum, and alloys thereof. The present invention more specifically relates to an anode assembly for cathodic protection wherein the performance of the sacrificial anode is enhanced by the use of a non-cementitious hydrophilic open-cell foam as a covering material for the sacrificial anode.

In this invention the open-cell foam covering material is impregnated by an activating salt intended to render the zinc anode in an electrochemically active state. The activating salt may be any one of several salts capable of breaking down the passive film that forms on the surface of the sacrificial anode material, thus preventing corrosion of the sacrificial anode material. Suitable activating salts for this purpose have been shown to be nitrates, and bromides, specifically lithium nitrate (LiNO₃) and lithium bromide (LiBr) and mixtures thereof.

It is further contemplated to immobilize the activating salt within the open-cell foam with an immobilizing agent designed to render the activating salt substantially stationary within the open cell foam. One immobilizing agent found to be especially advantageous is a gel, or semi-solid jelly-like material within the open-cell foam and substantially surrounding the sacrificial anode. The gel for use in the present invention has a degree of flexibility and flowability due to its significant water content. The gel of the present invention contains a significant amount of the activating salt. A preferred gelling agent for these salts is that known as Agar Agar, or simply Agar for short.

The anode assembly for cathodic protection also incorporates an elongated metallic conductor that serves to electrically connect the sacrificial anode to the reinforcing steel, or other metal to be protected, thereby providing an electrical path for the flow of protective current

The present invention also relates to a method of cathodic protection of reinforced concrete, and more particularly, to a method of improving the performance and service life of embedded anodes intended to apply cathodic protection to reinforcing steel and other metals embedded in concrete.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the present invention will become apparent to those skilled in the art to which the invention relates, with particular reference to the accompanying drawings, in which:

FIG. 1 is an elevational view in cross-section showing the cathodic protection system to which the present invention appertains; and

FIG. 2 is a graph showing protection current delivery versus duration in days.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates broadly to all reinforced concrete structures with which cathodic protection systems are useful. Generally, the reinforcing metal in a reinforced concrete structure is carbon steel. However, other ferrous-based metals can also be used.

The anode assembly of the present invention relates to galvanic cathodic protection (GCP), that is, cathodic protection utilizing anodes consisting of sacrificial metals such as zinc, aluminum, magnesium, or alloys thereof. Of these materials, zinc or zinc alloys are preferred for reasons of efficiency, longevity, driving potential and cost. Sacrificial metals are capable of providing protective current without the use of ancillary power supplies, since the reactions that take place during their use are thermodynamically favored. The sacrificial metal anodes may be of various geometric configurations, such as flat plate, expanded or perforated sheet, or cast shapes of various designs. A preferred configuration of the anode and anode assembly of the present invention is a high surface area configuration, such as an expanded metal mesh or a cast form with fins, protrusions, or the like, intended to increase the surface area of the anode. The actual surface area of the anode member preferably is from three to six times that of its superficial surface area. All elements of the anode should be metallurgically bonded to one another to comprise a single anode element within the anode assembly.

An important feature of the present invention is the use of non-cementitious open-cell hydrophilic foam substantially covering the sacrificial anode material. In this context, “non-cementitious” will be understood to mean that the foam is not impregnated with or encapsulated by a cementitious mortar in the manner described in the above-noted U.S. Pat. No. 7,160,433 to Bennett. Also, a non-cementitious open-cell hydrophilic foam will be understood to mean a non-cementitious open-cell foam which has the ability to provide a pathway for ionic conductivity from the sacrificial anode to the surrounding material. Preferably, this non-cementitious open-cell hydrophilic foam is compressible. This open-cell foam is in direct contact with the surface of the sacrificial anode, providing a pathway for ionic conductivity from the sacrificial anode to the surrounding material.

“Hydrophilic” means the ability to readily absorb and retain moisture. Also, within the scope of the present invention, it is important for the foam to provide a low resistance pathway for ionic conductivity. Impregnating the foam with a solution of a salt such as lithium nitrate or lithium bromide or a mixture of the two serves this purpose.

The non-cementitious hydrophilic open-cell foam may be naturally hydrophilic such as phenolic foam and other foams known as “floral foams,” which are used in the floral industry to maintain cut flowers moist and fresh. One source for such floral foams is Smithers Oasis. Such foams are desirable not only because they are naturally hydrophilic but also because they are capable of wicking water.

The non-cementitious hydrophilic open-cell foam can also be made from an open-cell foam which is not naturally hydrophilic but which is impregnated with a material which renders the non-cementitious open-cell foam hydrophilic. For example, the non-cementitious hydrophilic open-cell foam can also be made from a polystyrene foam which has be impregnated with an aqueous gel (i.e., a hydrogel) such as the agar agar and other hydrogels described below.

There are four commercial products marketed as discrete anodes used for the purpose of mitigation of corrosion of steel in reinforced concrete. All four of those products use a rigid cementitious-based material covering the anode. Although these products all provide a degree of protection against corrosion, the protective current they deliver is limited by the properties of the covering material. Rigid cementitious-based covering materials are characterized by inflexibility and very low diffusion coefficients. This has the result of limiting the mobility of ions in the covering material, causing excessive polarization, and therefore limiting protective current delivered. These materials also have the tendency to trap anode corrosion products against the anode surface, again resulting in polarization and limiting protective current. In cases of severe corrosion, the protective current delivered by these products may not be sufficient to control corrosion, resulting in a limited service life, resumption of corrosion, and eventual cracking and spalling of the concrete cover over the reinforcing steel. In some cases, cracking and spalling may resume after only very few years. It has been desired to increase the protective current delivery, and thereby to improve the protection against corrosion and extend the working service life of these products. This goal is, to large degree, achieved by the present invention, because the material surrounding the sacrificial anode is made from an open-cell foam which is non-cementitious, hydrophilic and desirably compressible as well.

In this invention the open-cell foam covering material is impregnated by an activating salt intended to render the zinc anode in an electrochemically active state. The activating salt may be any one of several salts capable of breaking down the passive film that forms on the surface of the sacrificial anode material, thus preventing corrosion of the sacrificial anode material. Salts known to be effective for this purpose include chlorides, acetates, iodides, fluorides, hydroxides, bromides, nitrates, phosphates, phosphites, and chlorates, as well as other less common salts. The best activating salts for this purpose have been shown to be nitrates, and bromides, especially lithium nitrate (LiNO₃) and lithium bromide (LiBr) and combinations thereof. The activating salts must be in amount sufficient to maintain the sacrificial anode material active, and in amount such that diffusion of activating salt away from the anode will not cause its concentration to drop below an effective amount. Generally, the activating salt should be present in an amount of at least 0.05 up to about 1 gram per cubic centimeter of the open-cell foam. The activating salt is typically added to the open-cell foam as an aqueous solution. This is particularly advantageous since the preferred lithium salts are both readily available and economically viable when supplied as aqueous solutions.

An additional feature of the present invention is the use of an immobilizing agent designed to render the activating salt substantially stationary within the open cell foam, thus preventing the activating agent from diffusion away from the sacrificial anode. One immobilizing agent found to be especially advantageous is a gel, or semi-solid jelly-like material within the open-cell foam and substantially surrounding the sacrificial anode. Gels are mostly liquid by weight, yet they behave like solids due to a three-dimensional cross-linked network within the liquid. Gels may have properties ranging from very soft and weak to hard and tough, but for the purposes of the present invention the gel used as an immobilizing agent within the open-cell foam is a jelly-like substance having a degree of flexibility and flowability. Preferred are water-based gels.

It has been found that, in order to achieve maximum long-term performance, the gel material should have a certain range of viscosity. If the covering material is too rigid, anode corrosion materials will be trapped against the anode surface and polarization will limit the protective current delivered to the steel, as is the case with cementitious covering materials. If the gel material covering the anode is too fluid, as a liquid, then it will be difficult to contain and the activating salt may migrate away from the anode surface. It has been found that, for optimum performance, the gel material covering the anode must therefore have a certain range of viscosity.

A fluid's internal resistance to flow, which is a measure of fluid friction, is its viscosity. Viscosity, which can be measured using various types of viscometers, may be thought of as the “thickness” or “internal friction” of a fluid. The SI physical unit of dynamic viscosity is the pascal-second (Pa·s). If a fluid with a viscosity of 1.0 Pa·s is placed between two plates, and one plate is pushed sideways with a shear stress of one pascal, it will move a distance equal to the thickness of the layer between the plates in one second.

It has been found in the present invention that for enhanced performance, the gel material covering the anode should have a viscosity between about 1 and 500 Pa·s. This is a viscosity ranging from about that of honey to about that of peanut butter. For a gel of this viscosity, ionic mobility will be relatively high, allowing the anode to operate with minimum polarization.

Common ingredients that can be used to promote gelling include polyvinyl alcohol, acrylate polymers and copolymers with an abundance of hydrophilic groups. The anode covering material also incorporates a strong activating agent (discussed above), and these may be particularly difficult to gel. In such cases it may be particularly advantageous to use a gelling agent known as agar-agar, or simply agar. Agar is a gelatinous substance derived from a polysaccharide obtained from the cell walls of some species of red algae. It is unofficially known as the “queen of gelling agents”. Chemically, it is a mixture of two components: the linear polysaccharide agarose, and a heterogeneous mixture of smaller molecules called agaropectin. Agar is the preferred gelling agent for this application. The gel, if used, may be incorporated into the sponge along with the activating agent.

The anode assembly for cathodic protection also incorporates an elongated metallic conductor such as insulated #16 AWG copper wire that serves to electrically connect the sacrificial anode to the reinforcing steel, or other metal to be protected, thereby providing an electrical path for the flow of protective current. The elongated metallic conductor may be attached to the reinforcing steel by one of several methods, such as wrapping, twisting, resistance welding, tig welding, mechanical compression and the like.

A system that utilizes the anode assembly described above is shown in FIG. 1. In this case , one end of the elongated metallic conductors 20 is electrically joined to a sacrificial metal anode 10, and the other end is wrapped securely around or otherwise coupled to a reinforcing bar 18 that has been cleaned to bright metal. The anode assembly is covered with or embedded in an open cell foam 12 as previously described. Together with the reinforcing bars and surrounding excavation, the anode assembly is then embedded in mortar or concrete 22. As soon as the assembly is positioned, the circuit is completed and protective current will begin to flow to the reinforcing bar in the vicinity of the anode assembly, thus imparting a degree of cathodic protection and mitigating corrosion of the steel. It is particularly desirable to cathodically protect the steel in the original concrete outside the patch area, thereby preventing the so called “halo effect,” or “anode ring effect.” In this manner, the service life of the concrete patch can be greatly extended.

FIG. 2 is a line graph showing the protective current delivered by Sentinel-GL, a commercial product provided by The Euclid Chemical Company (labeled in FIG. 2 as “Performance without Foam”) to the protective current delivered by an anode assembly of the present invention (labeled in FIG. 2 as “Performance with Foam”) The advantage of the present invention is shown by an increased level of protective current.

From the above description of the invention, those skilled in the art will perceive improvements, changes, and modifications. Such improvements, changes, and modifications within the skill of the art are intended to be covered by the appended claims presented below.

Having described the invention, the following is claimed: 

What is claimed is:
 1. An anode assembly for galvanic cathodic protection of a reinforced concrete structure comprising: at least one sacrificial anode member; a covering material consisting of a hydrophilic non-cementitious open-cell foam substantially covering the sacrificial anode member; an activating salt designed to keep the sacrificial anode in an electrochemically active state within the open-cell foam covering material; at least one elongated metallic conductor metallurgically bonded to the sacrificial anode.
 2. An anode assembly of claim 1 wherein the sacrificial anode member is zinc or a zinc alloy.
 3. An anode assembly of claim 1 wherein the sacrificial anode member is a high surface area configuration having an actual surface area from three to six times that of its superficial surface area.
 4. An anode assembly of claim 1 wherein the hydrophilic open-cell foam is a phenolic resin.
 5. An anode assembly of claim 1 wherein the hydrophilic open-cell foam is compressible.
 6. An anode assembly of claim 1 wherein the activating salt within the hydrophilic open-cell foam is a deliquescent or hygroscopic material.
 7. An anode assembly of claim 1 wherein the activating salt within the hydrophilic open-cell foam is lithium nitrate, lithium bromide, or combinations thereof.
 8. An anode assembly of claim 1 wherein the activating salt within the hydrophilic open-cell foam is present in the amount of between about 0.05 grams and about 1 gram per cubic centimeter.
 9. An anode assembly of claim 1 wherein the hydrophilic open-cell foam is impregnated with a gel.
 10. An anode assembly of claim 9 wherein the gel has a viscosity ranging from 1 to 500 pascal-seconds (Pa·s).
 11. An anode assembly of claim 9 wherein the gel is based on the agent agar-agar.
 12. An method for galvanic cathodic protection of a reinforced concrete structure, said method including: providing at least one sacrificial anode member; substantially covering the sacrificial anode member with a covering material consisting of a hydrophilic non-cementitious open-cell foam; impregnating the open cell foam, either before or after substantially covering the anode member, with an activating salt designed to keep the sacrificial anode in an electrochemically active state within the open-cell foam covering material; metallurgically bonding at least one elongated metallic conductor to the sacrificial anode, and; connecting the elongated metallic conductor to a ferrous reinforcing member within surrounding concrete, thus allowing protective current to flow.
 13. The method according to claim 12 utilizing an open-cell foam comprising a phenolic resin.
 14. The method according to claim 12 wherein the open-cell foam is compressible.
 15. The method according to claim 12 utilizing an activating agent selected from the group consisting of lithium nitrate, lithium bromide and mixtures thereof in an amount of between about 0.05 gram and about 1 gram per cubic centimeter of foam.
 16. The method according to claim 15 further impregnating the foam with a gel.
 17. The method according to claim 16 wherein the gel is based on the agent agar agar.
 18. The method according to claim 16 wherein the gel has a viscosity ranging from about 1 to about 500 pascal-seconds (Pa·s).
 19. The method according to claim 12 wherein the activating agent is a hygroscopic or deliquescent material. 